Resistance to antiretroviral therapy (ART) poses significant barriers to effective treatment of HIV infection and optimal long-term clinical care outcomes; however, there are few longitudinal data on rates of and factors associated with viral failure and resistance development in children, particularly in sub-Saharan Africa, where over 90% of HIV-infected children live. The long-term goal of our research team is to provide new evidence to improve the clinical disease management of HIV-infected children and adolescents in resource-limited settings. The purpose of this proposal is to longitudinally investigate treatment failure, drug resistance and associated factors among a previously established, carefully characterized cohort of 685 HIV-infected children in western Kenya. This work will be conducted within a long-standing US-Kenya partnership, the Academic Model Providing Access to Healthcare (AMPATH), which currently cares for over 70,000 adult and pediatric HIV- infected individuals at 65 clinical sites in Kenya. We will utilize the excellent AMPATH infrastructure, as well as a previous NIH-funded study that established this cohort of children (Vreeman: 1K23MH087225), to provide novel and significant data on the development of failure and resistance over five years. The Specific Aims of the proposal are: Aim 1: Determine the prevalence of viral failure and examine drug resistance mutation patterns among a retrospective study cohort of 685 perinatally HIV-infected Kenyan children on 1st-line ART. Aim 2: Investigate associations between specific adherence patterns, ART drug levels and other demographic and clinical factors, with viral failure and drug resistance; Aim 3: Study long-term immunologic, virologic and drug resistance outcomes and their associations in prospectively re-enrolled study participants; and Aim 4: Enhance analyses of viral failure, drug resistance accumulation and associated demographic and clinical factors by examining the longitudinal banked samples available for a subset of the study cohort (n=327). To achieve these aims, we will use previously collected, banked blood specimens of the study cohort (collected between May 2010-October 2013), along with detailed adherence data from electronic dose monitoring and drug concentrations; re-enroll as many of the study cohort as possible for an additional blood draw; assess long-term treatment failure and drug resistance using novel resistance testing technologies; use the established AMPATH Medical Records System to obtain all clinical and pharmacy data for each child from enrollment in AMPATH to present to examine longitudinal associations with failure and resistance; and conduct prospective electronic dose monitoring of current adherence for a subset of the cohort. This research will significantly impact understanding of longitudinal failure and resistance development and which factors influence their development. This proposal is unique because we propose: (1) longitudinal investigation of failure and resistance; (2) innovative approaches to drug resistance testing; and (3) detailed investigation of adherence, drug concentration and other clinical data, to impact pediatric HIV care in resource limited settings.